1. Field of the Invention
The present invention generally relates to automatic control systems for vehicles. More particularly, the invention relates to systems for controlling the handling characteristics of a vehicle (i.e., steering angle and wheel torque).
2. State of the Art
Presently, automatic control systems exist for stabilizing vehicle control. One area of automated vehicle control has been the development of automatic anti-skid braking systems to improve vehicle stability. In these systems, the vehicle's predominant safety device is regulated by computer control to, for example, account for emergency maneuvers (e.g., counter undesirable vehicle yaw motion). This computer control affords a consistent and dependable response during braking. Other examples of automated control systems typically used in vehicles include active and semi-active suspensions and vehicle traction control systems.
Presently, the control strategies for these active systems are not based upon control theory. Rather they are developed based upon intuitive analysis of the system being controlled with only limited use of feedback.
For example, the control strategies used to implement known anti-skid systems are based on intuitive analysis of braking system operation under various vehicle braking conditions. Further, these systems depend on empirically derived data and are therefore limited to specified conditions which have been evaluated in advance.
U.S. Pat. No. 5,001,636 represents a known control system which further provides yaw rate control for a vehicle during both braking and non-braking conditions. Limited use of feedback is used to detect whether yaw rate has exceeded a predetermined value. Sensors are used to detect steering wheel angle and vehicle speed. A reference yaw rate generator is provided which generates a reference yaw rate in response to the steering wheel angle and detected speed to control yaw motion of the vehicle. A yaw rate error is then used to provide limited yaw control by, for example, adjusting engine output torque.
Other known automated vehicle control systems control vehicle steering in reliance upon human responsiveness. Human response is often relied upon in known power steering systems to control forces applied to the steering wheel. Limited use of feedback in these systems merely permits the vehicle to respond rapidly to the driver's commands.
For example, U.S. Pat. No. 4,320,812 relates to a flow control system for use in a power steering system for amplifying manual steering torque. U.S. Pat. No. 4,992,944 also discloses an electrically-operated power steering control system. Here, a motor is controlled according to a predetermined function of the steering power using a conduction ratio based on torque variation and steering angle rate of change.
Although independent systems exist for providing limited yaw control and steering control, it would be desirable to provide integrated steering and wheel torque control, taking vehicle dynamics into account. Further, it would be desirable to provide a theoretically based controller which optimally controls actuators associated with these features.